merwin: azurite and alamosite 253 



is the essential equiv^alent of the Alum Bluf!" formation, if the 

 basal fossiliferous marl exposed at White Springs be included 

 with the Hawthorn. In western Florida the formation rests 

 upon the Chattahoochee formation, while in central Florida, 

 from Gainesville and Alachua southward, it immediately overlies 

 the Ocala limestone. 



Although the term "Hawthorn beds" was proposed in 1892 

 and Alum Bluff in 1894, it seems better to suppress Hawthorn 

 and adopt Alum Bluff, as the latter term has been extensively 

 applied in western Florida and the formation traced across 

 southern Georgia and thence northeastward to Savannah River. 



MINERALOGY. — The optical properties of azurite and alamosite. 

 H. E. Merwin, Geophysical Laboratory. 



Azurite. The chief optical properties of azurite, except the 

 refractive indices, were described by Des Cloizeaux.^ I have 

 measured microscopically the refractive indices and 2V of crys- 

 tals from Broken Hills, Australia, and from Butte, Montana. 

 The observed values for the two specimens do not differ more 

 than as -indicated. For Na-light a = 1.730 ±0.002, ^ = 1.758 ± 

 0.003, 7 = 1.838 ±0.003. 



Very thin sections transmit sufficient Na-light to permit a 

 fairly satisfactory measurement of 2V by the method based 

 upon the curvature of a zero isogyre. Three measurements varied 

 between 62° and 70? The optic axis of each of these sections 

 could be located within 0.75? Such location was not measur- 

 ably different for the greenish-blue light transmitted by the sec- 

 tion in white light. From one thick section showing an optic 

 axis and the acute bisectrix an accurate measurement of 2V 

 gave the value 68? Very thin sections show slight horizontal 

 dispersion. With decreasing wave-length the refractive index 

 increases rapidly. The following values were observed: at 671 ^^ 

 a = 1.719, at 589M/i « = 1.730, at 486mm « = 1.756. 



Since 2V for yellow and blue are nearly equal, the observed 

 angle, 2E = 151°, obtained by Des Cloizeaux may be reduced to 



1 Manuel de Mineralogie 2 : 194. 1874. 



